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Abstract:

An integrated, multidisciplinary basin analysis has been carried out for an approximately isochronous stratal interval of the Silurian fill of the Welsh Basin. The selected interval is the Upper Llandovery griestoniensis Chronozone which represents a time interval of probably less than one million years. Two mapped areas with contrasting structural styles illustrate the heterogeneous response of the basin-fill to basin inversion and shortening. One major Welsh structure, the Central Wales Synclinorium, is shown to be related to a long-lived structural discontinuity which significantly influenced early Silurian sedimentation patterns. During extension/transtension in the basin this structure, as the edge of a tilted fault block, formed a palaeohigh. Turbidite systems were confined to either side and were ponded in relatively small-scale sub-basins defined by the tilted fault block geometry. In basinal facies of griestoniensis Zone age four major lithofacies associations are recognized: the channel-lobe transition, sandstone lobe, lobe fringe, and lateral basin-slope facies associations. They represent deposits of the outer area of a moderately large (minimum length of 100 km), elongate turbidite system and its bounding slope. Large scale erosional structures, filled with conglomerates and pebbly sandstones, occur in the more proximal areas of exposure. They are either channels or huge scours excavated by large volume turbidity currents undergoing hydraulic jumps. Lateral facies variations in sandstone lobes reflect down-system and cross-system variations in competence and capacity of flows. Palaeocurrent patterns and inferred onlap relationships reflect confinement by structurally-controlled slopes. Since the system developed during a time of widespread transgression sandstone lobes probably record pulses of rapid tectonic uplift at source. Unconformities, corresponding to times of active growth of depositional sandstone lobes in the basin, were generated on the southern basin margin. Lateral gradients in palaeo-oxygenation of bottom waters and fluctuations in oxicity through time are reconstructed using trace fossil evidence and diagenetic mineral assemblages. Relic downward-migrating oxidation fronts are recorded for the first time from ancient turbidites. A diverse trace fossil assemblage associated with the turbidite system is described and interpreted. Detailed logging through the basinal sequence reveals that times of dysaerobic bottom water conditions, during which turbidites and hemipelagites were reworked by soft-bodied infauna, alternated with periods of bottom water anoxia, during which laminated hemipelagites containing a pelagic fauna of graptolites were preserved. Such alternations may be very rapid, suggesting close proximity of the anoxic-oxic interface to the sediment surface, or may have far longer periodicities, reflecting periods of stable pycnocline development above anoxic bottom waters. Phosphate cements are shown to have initiated very close to the sediment surface beneath oxygen-deficient bottom waters in the basin.